In general, a plated steel sheet is used for an outer plate of an automobile from the viewpoint of rust-proofing property, and an alloyed-zinc-plated steel sheet is mainly employed. The alloyed-zinc-plated steel sheet is a plated steel sheet whose weldability and corrosion resistance after coating are improved by performing an alloying treatment on a steel sheet subjected to zinc plating and causing Fe to be diffused from the steel sheet (the base steel sheet) into a plating layer. Meanwhile, in the case of the alloyed-zinc-plated steel sheet, the plating layer becomes harder due to the diffusion of Fe from the base steel sheet so that the plating layer is easily peeled off and there are also peculiar problems, which are not observed in a soft hot-dip zinc-plated steel sheet, such as powdering and flaking.
Regarding the plated steel sheet with a hard plating layer, the plating layer is easily cracked by an external pressure. Once cracks occur, the cracks are propagated to an interface between the plating layer and the base steel sheet, and thus the plating layer is peeled off from the interface and then dropped off. For example, when the alloyed-zinc-plated steel sheet is used for an outer plate of an automobile, coating and the plating layer are simultaneously peeled off by chipping due to striking of stones while the vehicle is moving, the base steel sheet is easily exposed, and thus corrosion becomes severe as compared to the soft plated steel sheet that is not alloyed, which is called chipping resistance decreasing.
In this regard, in order to improve chipping resistance of the outer plate of the automobile, it is preferable to use a plated steel sheet which has sufficient sacrificial corrosion resistance and includes a plating layer having high corrosion resistance against corrosion from a chipping portion is sufficiently suppressed as an outer plate. Simply, in order to improve chipping property, it is an easy way to use a soft plating layer, hot-dip Zn-plated steel sheet, or electro plated Zn-steel sheet; however, these plated steel sheets are rapidly corroded when a painted layer-peeled-off portion is generated by chipping. For this reason, these plated steel sheets are not fundamental solutions for chipping corrosion. When corrosion of the plated steel sheet progresses, a pitting corrosion portion becomes too larger, and result in red rust is generated from the center portion.
For example, as a plated steel sheet excellent in corrosion resistance, Patent Literature 1 discloses a Zn—Al—Mg—Si plated steel sheet and Patent Literature 2 discloses a hot-dip Zn—Mg-based alloy plated steel sheet. The plated steel sheet as described in Patent Literature 1 in which corrosion resistance is increased by adding various alloy elements such as Al, Mg, and Si to a plating layer is a Zn-based plated steel sheet. A chipping portion is less generated in a relatively soft plating layer, but when the chipping portion or coating-peeled-off portion is generated, pitting corrosion rapidly progresses as was expected. When corrosion progresses, red rust is generated from the center portion as was expected, and thus sufficient sacrificial corrosion protection property is not obtained. For these reasons, it cannot be said that the plated steel sheet in which corrosion resistance is improved on the basis of Patent Literature 1 has sufficient chipping resistance.
As a means for solving the above-described problems, there is a Zn—Mg-based alloy plated steel sheet as described in Patent Literature 2. Since a large amount of Mg is contained in the plating layer, sacrificial corrosion protection performance is sufficient, and even when pitting corrosion progresses, this plated steel sheet is an effective plated steel sheet in which red rust is not generated for a long period of time. However, in the plated steel sheet described in Patent Literature 2, Zn3Mg7 is originally formed in the plating layer to extremely increase hardness and too many pitting corrosion portions are generated so that chipping resistance is significantly decreased.
In this regard, in order to achieve the balance between corrosion resistance and chipping resistance, a means for configuring the plating layer with a plurality of layers and compositing the plurality of layers has been proposed. For example, Patent Literature 3 discloses a method for forming a hard Mg—Al alloy plating layer having high corrosion resistance on the surface of a soft low-alloy plating layer (an alloyed-zinc plating layer) formed on a steel sheet by an ion plating method. When the plating layer is multi-layered by laminating a soft low-alloy plating layer and a hard Mg—Al alloy plating layer having high corrosion resistance, there is a possibility that the balance between corrosion resistance and chipping resistance can be achieved. However, actually, since the low-alloy plating layer serving as a base layer of the plating layer is a simple alloyed-zinc plating layer, the plated steel sheet is inferior to the alloy plated steel sheets described in Patent Literature 1 and Patent Literature 2 in corrosion resistance. Further, in order to obtain corrosion resistance similar to corrosion resistance described in Patent Literatures 1 and 2 in the plated steel sheet in which the plating layer is multi-layered as disclosed in Patent Literature 3, it is necessary to considerably increase the thickness of the Mg—Al alloy plating layer of an upper layer formed by ion plating. However, it is difficult to produce a plated steel sheet with high corrosion resistance and excellent chipping resistance by the method of increasing the thickness through the ion plating method. In addition, it is necessary to provide a two-step process of alloyed-zinc plating and ion plating, and thus there is also a problem of an increase in cost.
As described above, hitherto, a means for achieving the balance between corrosion resistance and chipping resistance has not been disclosed in the alloy-based plated steel sheet which is added with a relatively large amount of alloy in order to improve corrosion resistance.
Incidentally, the quasicrystal is a crystal structure first found by Daniel Shechtman in 1982 and has icosahedron atomic arrangement. This crystal structure is an aperiodic crystal structure having a peculiar revolution symmetry (for example, fivefold symmetry) that is not obtained in general metal and alloy and is known as a crystal structure equivalent to an aperiodic structure typified by a three-dimensional Penrose pattern.
Since finding of the new arrangement of metal atoms (that is, the new crystal structure), a quasicrystal having a quasi-periodic structure and a peculiar revolution symmetry has been attracting attention. In recent years, it is found out that the quasicrystal can be obtained by crystal growth, but hitherto, the method for producing a quasicrystal is generally a liquid quenching method. For this reason, the shape of the quasicrystal is limited to powder, foil, small pieces, and thus there are remarkably few practical examples of products using the quasicrystal.
Patent Literature 4 and Patent Literature 5 disclose a high strength Mg-based alloy and a production method therefor. These Mg-based alloys are alloys which are obtained by dispersing and precipitating a hard quasicrystal phase having a particle size of several tens of nm to several hundreds of nm in a metallic structure and are excellent in strength and elongation. In these Patent Literature 4 and Patent Literature 5, the feature that the quasicrystal is hard is utilized.
Further, Patent Literature 6 discloses a thermoelectric material using an Al-based quasicrystal. In this Patent Literature 6, the feature that the quasicrystal is excellent in thermoelectric property is utilized. Patent Literature 7 discloses a heat-resistant catalyst prepared by using a quasicrystal Al alloy (an Al-based quasicrystal) as a precursor and a production method therefor. In this Patent Literature 7, the feature that the quasicrystal having no periodic crystal structure is brittle and is easily crushed is utilized. As such, in the inventions of the related arts, in many cases, the quasicrystals are dispersed as fine particles or the quasicrystals that are fine particles are solidified and molded.
As a utilization form separate from these inventions, Patent Literature 8 discloses metal coating for a utensil for cooking food products, the coating containing a quasicrystal. In this Patent Literature 8, coating that is excellent in abrasion resistance and corrosion resistance to a dietary salt is applied to a utensil for cooking food products by plasma spraying alloy powder containing a quasicrystal, which is consisting of Al, Fe, and Cr and is excellent in corrosion resistance.
As described above, the Mg-based quasicrystal is used as a material excellent in strength and the Al-based quasicrystal is used as a member excellent in strength, a thermoelectric material, coating for a utensil for cooking food products, or the like. However, these utilizations are limited and it cannot necessarily be said that the quasicrystal is used in various fields.
The quasicrystal has an excellent performance derived from a unique crystal structure. However, this characteristic is only partially revealed, and thus currently, it cannot be said that the quasicrystal is a material widely used in industrial fields. The present inventor attempted to apply a quasicrystal, which has not been industrially used yet, to a plating layer of a plated steel sheet and to improve both of corrosion resistance and chipping resistance.